06 - Chemistry 2000 Lecture 6: Applications of valence bond...

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Chemistry 2000 Lecture 6: Applications of valence bond theory Marc R. Roussel
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Ethene C C H H H H I Trigonal planar carbons = sp 2 hybridization I sp 2 hybrids used to make sigma bonds to H atoms (with their 1s orbitals) and between the C atoms: C H H H H C sp 2 -sp 2 valence bond 1s-sp 2 valence bond
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I This leaves one unused p orbital on each carbon atom: C C H H H H I These p orbitals are combined into a π valence bond: ψ π ( r 1 , r 2 ) = 2 p , A ( r 1 )2 p , B ( r 2 ) + 2 p , A ( r 2 )2 p , B ( r 1 ) Important note: We can only form the π bond if the two p orbitals are aligned. Put another way, the π bond will be broken if we twist the molecule, which takes a lot of energy. Implication: Molecular fragments containing π bonds are strongly constrained to be planar (or nearly so). I We could have reached this conclusion based on similar reasoning from MO theory as well.
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An aside: sigma-pi separation I We have so far treated (in both MO and VB theory) σ and π bonds as if they are entirely separate. I
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This note was uploaded on 03/03/2012 for the course CHEM 2000 taught by Professor Roussel during the Fall '06 term at Lethbridge College.

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06 - Chemistry 2000 Lecture 6: Applications of valence bond...

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